In catalytic cracking processes hydrocarbon feedstock is injected into the riser section of a hydrocarbon cracking reactor, where it cracks into lighter, valuable products on contacting hot catalyst circulated to the riser-reactor from a catalyst regenerator vessel. As the endothermic cracking reactions take place, the catalyst gets covered with coke deposits. The catalyst and hydrocarbon vapors are carried up the riser to the disengagement section of the reactor, where they are separated. Subsequently, the catalyst flows into the stripping section, where the hydrocarbon vapors entrained with the catalyst are stripped by steam injection, and the stripped catalyst flows through a spent catalyst standpipe and into the catalyst regenerator vessel.
Typically, the catalyst is regenerated by introducing air into the regenerator vessel to burn coke off the catalyst, thereby rejuvenating it. The coke combustion reactions are highly exothermic and heat the catalyst. The hot, reactivated catalyst flows through the regenerated catalyst standpipe back to the riser to complete the catalyst cycle. The coke combustion exhaust gas stream rises to the top of the regenerator and leaves the regenerator through the regenerator flue. The exhaust gas contains nitrogen and carbon dioxide (CO.sub.2), and generally also contains carbon monoxide (CO), oxygen, sulfur oxides (SO.sub.x), nitrogen oxides (NO.sub.x) and reduced nitrogen species, such as ammonia.
The catalyst regenerator may be operated in complete combustion mode, which has now become the standard combustion mode, or in partial CO combustion mode. In complete combustion operation, the coke on the catalyst is completely burned to CO.sub.2. This is typically accomplished by conducting the regeneration in the presence of excess oxygen, provided in the form of excess air. The exhaust gas from complete combustion operations comprises primarily CO.sub.2, nitrogen and excess oxygen, but also contains NO.sub.x and SO.sub.x.
In partial carbon monoxide combustion mode operation, the catalyst regenerator is operated with insufficient air to burn all of the coke in the catalyst to CO.sub.2, consequently the coke is combusted to a mixture of CO and CO.sub.2. The CO is oxidized to CO.sub.2 in a downstream CO boiler. The effluent from the CO boiler comprises primarily CO.sub.2 and nitrogen, but also contains NO.sub.x and SO.sub.x.
Recently, there has been considerable concern about the amount of NO.sub.x and SO.sub.x being released to the environment in refinery flue gases. It is now the accepted view that most of the NO.sub.x present in catalyst regenerator exhaust comes from coke nitrogen, i.e. nitrogen contained in the coke in the form of heterocompounds such as condensed cyclic compounds, and that little or none of the NO.sub.x contained in the exhaust gas is derived from the nitrogen contained in the air feed to the regenerator. The mechanism by which the coke nitrogen ends up as NO.sub.x differs, depending on whether the regenerator is operated in complete combustion mode or in partial combustion mode. In complete combustion mode regenerator operation, coke nitrogen is converted to a mixture of NO.sub.x and elemental nitrogen. In this operational mode, the amount of NO.sub.x in catalyst regenerator flue gas tends to increase as the excess oxygen concentration from the regenerator increases.
When the regenerator is operated in partial CO combustion mode, very little NO.sub.x is produced in the regenerator, and coke nitrogen leaves the regenerator as reduced nitrogen species, such as ammonia. The reduced nitrogen species are unstable in the CO boiler, and they are easily converted to NO.sub.x and elemental nitrogen.
Several approaches have been used in industry to reduce NO.sub.x in cracking catalyst regenerator exhaust gases. These include capital-intensive and expensive options, such as pretreatment of reactor feed with hydrogen and flue gas post-treatment options; intermediate cost options, such as split-feed injection to the hydrocarbon reactor, and less expensive options, such as the use of catalysts and catalyst additives.
Efforts to reduce NO.sub.x from boiler stacks downstream of FCC units operated in partial combustion mode are centered on the reduction of ammonia and other NO.sub.x precursors in regenerator flue gas. U.S. Pat. No. 4,755,282 discloses the use of a noble metal on an inorganic support to reduce the ammonia content of flue gas from regenerators. U.S. Pat. No. 4,744,962 teaches the addition of NO.sub.x either to the regenerator or to the downstream flue gas line. U.S. Pat. No. 5,021,144 teaches the reduction of ammonia from a regenerator operated in partial CO combustion, by the addition of excess amounts of CO promoter.
U.S. Pat. No. 5,268,089 teaches that NO.sub.x can be reduced by regenerator operation "on the brink", i.e. in a region between conventional partial CO combustion operation and complete combustion operation with less than 0.05 mol % excess CO. The patent indicates that by operating in this mode, the reduced nitrogen species, such as ammonia, which are formed in partial CO combustion operation, are oxidized to nitrogen oxides and elemental nitrogen, and because of the prevailing reducing environment in the regenerator, the nitrogen oxides are reduced to elemental nitrogen prior to leaving the regenerator. Drawbacks associated with operating in the above-described mode are the existence of very high regenerator temperatures and afterburn, in addition to the difficulties associated with regenerator controllability.
Several patents disclose the reduction of NO.sub.x in FCC regenerators by means of promoters, segregated feed cracking, post treatment of exhaust gas, etc. These patents are discussed in detail in U.S. Pat. No. 5,268,089, the disclosure of which is incorporated herein by reference.
Because of considerable pressure from environmentalists and others to avoid polluting the atmosphere with noxious gases, efforts are continuously underway to find new and improved methods of reducing the concentration of NO.sub.x and SO.sub.x in industrial flue gases, such as FCC regenerator exhaust gases. This invention provides a method of taking advantage of the peculiar nitrogen chemistry in partial CO combustion to reduce the amount of effluent NO.sub.x by enriching selected zones in the regenerator with oxygen.